1. Field of the Invention
The present invention relates generally to the production of tubing by a combination of mechanical and thermal treatments and, more particularly, is concerned with enhancement of the radial texture of tubes composed of metallic materials, such as zirconium and alloys thereof, which have a hexagonal close-packed crystal structure, by insertion of a diametral expansion and a recrystallization anneal within an otherwise conventional sequence of intermediate diametral and wall thickness reductions and recrystallization anneals leading up to a final diametral and wall thickness reduction and final anneal for the production of such tubes.
2. Description of the Prior Art
The processing procedures applied to production of tubing composed of metallic materials, such as zirconium and alloys thereof, having a hexagonal close-packed crystal structure, conventionally consist of combinations of mechanical and thermal treatments. For instance, the mechanical treatments applied in the production of Zircaloy tubing are the cold deformations produced in tubing by multiple pilger reductions used to reduce the cross-sectional dimensions of the tubing. (A pilgering process produces axial elongation of a tube to a finished size over a stationary mandrel through effecting a reduction in both the diameter and wall thickness of the tube by means of two circumferentially grooved dies that embrace the tube from above and below and roll in a constant cycle back and forth along the tube.) The thermal treatments applied in the production of Zircaloy tubing are the vacuum annealing temperatures used for intermediate (between pilger reductions) and final (after the last pilger reduction) heat treatments. Below about 1000 degrees F., Zircaloy does not recrystallize (depending on the amount of cold work and time at temperature) and thus the heat treatment is termed a stress relief anneal. Above this temperature, it recrystallizes and the heat treatment is then a recrystallization anneal.
One conventional process sequence for production of Zircaloy tubing to be used as nuclear fuel cladding has four basic steps. The first three steps are called intermediate steps and the fourth step is termed the final step. Each step of the intermediate steps includes a pilger reduction pass followed by a recrystallization anneal at about 1250 degrees F. The final step includes a pilger reduction pass followed by a stress relief anneal at about 870 degrees F.
As mentioned above, the multiple pilger reduction passes are employed to elongate the tube by reducing its cross-sectional dimensions. Each reduction is characterized by the total deformation expressed as percent reduction in cross-sectional area and the distribution of this deformation between the radial and circumferential directions (deformation ratio). The deformation ratio (Q ratio) is commonly expressed as a ratio of percent wall reduction to outside diameter reduction. Typically, Q ratios greater than 1, especially in the last or final pilger reduction, are used to produce a textured Zircaloy product resistant to radial hydride formation in service.
Texture is an important property of Zircaloy tubes used as nuclear fuel cladding. It has a strong influence on other properties (mechanical and chemical) which are important to in-service performance of nuclear fuel. Texture in zirconium alloys is commonly determined by x-ray methods and measuring the Kearns parameter, "f.sub.r ". (For a more detailed discussion of the Kearns texture parameter, f.sub.r, attention is directed to a November 1965 report designated WAPD-TM-472 by J. J. Kearns entitled "Thermal Expansion and preferred Orientation in Zircaloy".) The Kearns texture parameter indicates the fraction of all basal poles present in a material that are effectively oriented in any of the three reference directions, radial (f.sub.r), circumferential (f.sub.rc) or axial directions (f.sub.ra), in a tube. The value of "f.sub.r " can vary between 0.0 and 1.0. In an isotropic, untextured material the value of the parameter would be 0.33. For Zircaloy nuclear fuel clad tubing, the Kearns radial texture parameter is usually greater than 0.5 with the basal poles oriented predominantly in the radial direction.
An alternative method of characterizing texture in Zircaloy tubing is to measure the anisotropy of plastic deformation using the contractile strain ratio (CSR) test. CSR is the ratio of circumferential (diameter) to radial (through wall) strains accompanying a small amount of axial elongation in a tensile test. Zircaloy tubes are usually textured with the basal poles generally oriented towards the radial direction. Furthermore, since the resistance to deformation is highest in the basal pole direction, the values of CSR measured in Zircaloy fuel clad tubing are greater than 1.0. CSR and the Kearns texture parameter, f.sub.r, both are indications of the degree of texture and they have been shown to be directly related to each other. (Reference: Van Swam, L. F. P. et al; "Relationship Between Contractible Strain Ratio R and Texture in Zirconium Alloy Tubing"; Metallurgical Transactions A, Volume 10A, pages 483-487. April 1979.)
A major factor in determining texture in Zircaloy is the direction of plastic deformation in the three principle directions (axial, circumferential and radial) produced during metalworking. The basal poles align in a plane normal to the direction of tensile or positive plastic deformation and parallel to the direction of greatest compressive or negative deformation. In pilgering, positive deformation takes place in the axial direction resulting, therefore, in the basal poles being oriented in the transverse plane defined by radial-circumferential directions, as seen in FIG. 1. Within the transverse plane, the basal poles tend to be further aligned in the direction of the greatest compressive deformation. In Zircaloy tube manufacturing, the relative amounts of compressive deformation in the radial and circumferential directions control the texture in the final product with a higher proportion of radial compressive deformation producing a more textured product.
The control of texture is a major concern in the development of processing procedures for Zircaloy nuclear fuel clad tubing. By conventional cold reduction in the pilgering process, the ratio of wall reduction to diameter reduction, the Q ratio, is the major controlling parameter in the texture and texture-related contractile strain ratio (CSR) property for the stress relieved zirconium tubing product. The Q ratio, or deformation ratio, is an indication of the relative distribution of deformation in the radial (through wall) to circumferential (diametral) directions produced during pilgering. Thus, the deformation pattern produced during metalworking of tubing is usually characterized by the Q ratio, the ratio of the radial (due to wall reduction) to circumferential (due to diameter reduction) deformations produced during pilgering. Generally, the higher the Q ratio produced in the pilgering operation the greater the radial orientation of the basal poles in the product.
Heretofore, the prevailing thinking within the industry was that the Q ratio of the final pilger reduction is the primary parameter controlling the texture and thus CSR in the zirconium tube product. However, while small changes in texture and CSR may occur with variations in final pilger Q ratio, significant changes have not been obtained and there is clearly other factors that must be considered.
Recent work leading to the present invention, but not forming part of the prior art, has shown that the Q ratio of the multiple pilger reductions during the intermediate steps rather than just that of the final pilger reduction is more important to texture control in stress relieved final products. This work demonstrates that the texture of the final Zircaloy product is much more sensitive to the total processing history than simply the final deformation processing. Texture of Zircaloy tubing is thus established by the combined or "effective" Q ratio of multiple pilger reductions rather than just that of the final pilger pass such that the texture of the material at the intermediate steps of processing has a direct effect on that of the final product.
While this work provides a basis for achieving higher texture and CSR in the final product, there is a limit to how much increase can be achieved in these properties by altering the pilger reduction schedule alone. All conventional metalworking processes for tubing (i.e., pilgering) necessarily consist of reductions in both wall and diameter and a corresponding axial elongation. There is, therefore, a maximum Q ratio that can be applied to the tube and still accomplish the overall objective of converting a large cross section tube extrusion to a small diameter, thin wall fuel clad tube.
Consequently, a need still exists to develop an alternative approach to increasing the texture of zirconium tubing products. Such approach would be one which avoids the necessity of major expenditures in tool design and manufacture to achieve higher Q ratios and product textures. This approach should also be capable of obtaining levels of texture significantly greater than that available by conventional metal working.